Radiator cap issues

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oldjunk

oldjunk

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Hey guys,
I recently am having issues holding pressure with the cooling system. My pressure tester will hold normal pressure with no problem. The issue is the cap. Every cap that i have used will not hold the normal pressure. Coolant will consistantly pour out the overflow. Any ideas as to why this is happening? The only thing I can come up with is the lip of the rad has worn away. Is is common? Any input would be gladly appreciated. Thanks guys.

-Oldjunk
 
Does the cap spring up? Is the inner part of the cap (inner rubber seal) pushing down with proper force on the radiator? Is the rubber seal crack free? Is the inner seat in the radiator smooth and ding free? If no to any questions then it will not seal. Are you using a pressure/vacuum cap or just a pressure cap?
 
what rad, i used a repop on my original 68 rad and it worked way better than the generic mopar cap that was on it. my 71 demon has an aftermarket rad and any new cap doesn't work only an old 25 yo stant
 
If EVERY repop cap that you have tried wont hold pressure then I would have to say that there is something odd about the radiator OR have you tried an original N-O-S?
 
I'm running the stock '72 340 radiator. I've tried the lever locking type and oem type made by stant. When I put my pressure tester on the rad, it builds up normal pressure (around 12-13 psi). If i put the aftermarket rad cap on, it seems like no pressure is building up at all. I can almost completely squeeze the upper and lower hoses. I am also loosing coolant out of my overflow. Sounds to me like there is some sort of sealing issue with these aftermarket caps. The rad is perfectly ok, except for this issue. Any idea on how I would remedy this?
 
Could you post some pics of the radiator and cap? That may help.
 
Stant has two versions of radiator caps. Just went through same problem with our 68 Barracuda. They have a version that they call "vented" - the small brass vacuuum vent will not be tight to the rubber seal with this style cap. We never got the cooling system to build pressure. We switched to a "non-vented" cap and it will hold pressure like nobodies business. An OEM style cap for our car is bilt like the non-vented cap, except that on the OEM there is no outter gasket for use with a fluid recovery system. That little seal disk is up tight to the vacuum vent.

Do not confuss vented and non-vented in this case with the "safety vented" which have a lever to release pressure without removing the cap.
 
Steve68 said:
Stant has two versions of radiator caps. Just went through same problem with our 68 Barracuda. They have a version that they call "vented" - the small brass vacuuum vent will not be tight to the rubber seal with this style cap. We never got the cooling system to build pressure. We switched to a "non-vented" cap and it will hold pressure like nobodies business. An OEM style cap for our car is bilt like the non-vented cap, except that on the OEM there is no outter gasket for use with a fluid recovery system. That little seal disk is up tight to the vacuum vent.

Do not confuss vented and non-vented in this case with the "safety vented" which have a lever to release pressure without removing the cap.
Click to expand...

Steve,
Sounds like this is my issue. The cap is indeed the vented type. Do you have the correct part # for the non-vented? I just went to the auto parts store and the guy there said the vented type was the only one available. Thanks for your help!
 
I don't keep the car at my house so I will try to get by and look at the cap tomorrow.
 
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